(fx= 12 13)
=> #f
(fx= 12 12)
=> #t
(fx= 16 (fx+ 13 3))
=> #t
(fx= 16 (fx+ 13 13))
=> #f
(fx= (fx+ 13 3) 16)
=> #t
(fx= (fx+ 13 13) 16)
=> #f